The present invention relates to a phase error detector useful for carrier recovery and phase noise suppression in digital VSB (vestigial sideband) and QAM (quadrature amplitude modulation) demodulators.
As an example of the prior art, FIG. 1 shows a block diagram of a phase error detector proposed for use in demodulators in high-definition digital television receivers receiving a VSB signal. The I-signal or in-phase signal and Q-signal or quadrature signal are multi-level digital signals obtained by synchronous detection of the received VSB signal, using a pair of recovered carrier signals with mutually orthogonal phase, or using a recovered carrier signal and a filter such as a Hilbert filter. The I-signal includes the transmitted data, referred to as transmitted symbols, the values of which are determined by a data decision circuit 1. A subtractor 2 takes the difference .delta.I between these values and the actual received levels of the I-signal. A divider 3 divides the difference (.delta.I) by the Q-signal, thereby generating a phase error signal (.theta.) indicating the phase error between the recovered carrier signal and the carrier component of the received signal.
FIG. 2 illustrates the operation of this phase error detector with reference to an orthogonal I-Q coordinate system. R(I, Q) denotes a received symbol, the I and Q coordinates being the received levels of the I-signal and Q-signal during the symbol interval. A phase error of .theta. in the recovered carrier signal rotates R(I, Q) by a phase angle of .theta. from the actual transmitted symbol T(I.sub.0, Q.sub.0), the rotation being clockwise with respect to the origin of the I-Q coordinate system. The I-coordinate of the received symbol, which should give the transmitted symbol value, is thereby moved by an amount .delta.I from the true value I.sub.0. Incorrect data will be obtained if .delta.I exceeds one-half the distance D between adjacent symbol levels on the I-axis. The phase error detector in FIG. 1 estimates the phase error .theta. as .delta.I/Q; that is, as the tangent of the angle .psi.. EQU .theta..apprxeq.tan .psi.=.delta.I/Q
In a carrier recovery circuit, the phase error detector forms part of a phase-locked loop (PLL) that corrects phase and frequency errors in the recovered carrier signal. In a phase noise suppression circuit, the phase error detector forms part of a PLL that corrects residual phase noise in the demodulated signals.
FIG. 3 shows a block diagram of a type of PLL used for both of these purposes. In carrier recovery, I.sub.R and Q.sub.R are the in-phase and quadrature components of a partially demodulated digital signal produced by semi-synchronous detection of the received signal with a recovered carrier signal that only approximately matches the received carrier phase and frequency. A complex multiplier 17 multiplies I.sub.R and Q.sub.R by a complex-valued signal to produce the I-signal and Q-signal input to the phase error detector 18. The phase error detector 18 generates a phase error signal .theta.. The loop filter 19 filters the phase error signal .theta. to remove unwanted high-frequency components while providing a suitable gain. The complex carrier generator 20 receives the filtered phase error signal and generates the complex-valued signal mentioned above, which is a digital signal having sine and cosine components. The phase and frequency of the complex-valued signal are adjusted according to the filtered phase error so that they match the phase and frequency error of the recovered carrier signal, enabling the complex multiplier 17 to complete the demodulation process. In a phase noise suppressor, I.sub.R and Q.sub.R are the completely demodulated in-phase and quadrature signals, and the complex-valued signal comprises the sine and cosine of the filtered phase error.
When the phase error detector shown in FIG. 1 functions as the phase error detector 18 in FIG. 3, its performance directly affects the error rate of the received data. A problem with the phase error detector in FIG. 1 is that the accuracy of the estimated phase error .theta. depends on the transmitted data. For a given real phase error, different transmitted data values can lead to very different error estimates. FIG. 4 shows an example in which transmitted symbols T.sub.A (I.sub.0A, Q.sub.0) and T.sub.B (I.sub.0B, Q.sub.0) are both received with identical phase errors. For simplicity, both have the same Q-coordinate Q.sub.0. Application of the above approximation to these received symbols produces one estimate (tan t.psi..sub.A) for the phase error (.theta.) of R.sub.A (I.sub.A, Q.sub.A), and a much larger estimate (tan.psi..sub.B) for the same phase error (.theta.) of R.sub.B (I.sub.B, Q.sub.B). The difference arises because the angle .psi. is a function of the transmitted data value I.sub.0, as shown by the following equation. EQU .psi.=tan.sup.-1 {tan(.theta.)+[1/(Q cos .theta.)-1/Q]I.sub.0 }
Depending on the transmitted data, the prior art can lead to erratic PLL behavior, with adverse effects on carrier recovery and noise suppression.
In a QAM demodulator, both the I-signal and Q-signal include transmitted data. In addition to the problem described above, the phase error detector in FIG. 1 has the further problem of ignoring the independent information content of the Q-signal.